March 9, 2010

Recent Earthquakes Rock Public Awareness

On Feb. 27, when Haiti laid in shambles from a magnitude 7.0 earthquake that leveled the nation less than two months prior, a magnitude 8.0 earthquake struck Chile. The quick succession of these two calamities jolted public conscience of earthquakes, rekindled curiosity about forecasting and generated speculation about the relationship between natural disasters and climate change.

Three components comprise an earthquake forecast: location, time and magnitude. Geologists predict these components with varying degrees of confidence. “We know very well where earthquakes occur,” said Prof. Rick Allmendinger, earth and atmospheric sciences, who has conducted field research in Chile since 1981.

The Haitian and Chilean earthquakes occurred along plate boundaries. In Haiti, two plates are sliding past each other, and down the west coast of Chile, the oceanic plate is subducting below the continental plate. In “subduction,” one plates to force another deeper into the Earth’s crust.

Consequently, both Haiti and Chile are part of “very discrete belts that continuously produce earthquakes,” he said.

In fact, if all the recorded earthquakes along the Chilean coastline were chronologically projected onto a map, it would be clear that for the past 150 years, earthquakes have successively been filling in gaps along the coastline.

The Iquique Gap in northern Chile, where Allmendinger conducts field research, is the last gap in the current cycle. It is not if but when the next will strike. Geologists can predict with a degree of certainty that within the next few decades, an earthquake will occur in the Iquique Gap.

Forecasters are not able to provide narrower time frames. When passing plates overcome their countering friction, they slip past each other and induce an earthquake, Allmendinger said. A level of tectonic stress will inevitably overcome the friction, causing a sudden slip and producing an earthquake. Geologists do not know the exact level of stress for any particular fault.

The surface area of the fault plane determines the amount of slip, and the amount of the slip determines magnitude. In Haiti, two plates are sliding past each other along a vertical Strike Split fault plane. In Chile, one plate is sliding underneath another along a gently inclined Subduction Megathrust fault plane. From the surface down through the part of the crust in which plates slide frictionally, the gently inclined plane of Chile covers more surface area than the vertical fault plane of Haiti. Consequently, the Haitian earthquake had a slip of about 5 meters, and recent estimates predict a slip of 10 to 15 meters for the Chilean earthquake.

Since magnitude is determined by surface area and the average slip, each fault plane generates earthquakes of roughly consistent size, explained Allmendinger. Using this knowledge, and the size of the gap, geologists are able to predict the magnitude of an earthquake.

Although the timely occurrences of the Chilean and the Haitian earthquakes gave the perception of an abnormally high disaster frequency, the earthquakes were actually perfectly normal events in their respective cycles. The cycles are natural ­– not a consequence of climate change.

“While they were large and damaging, they were not in any way out of the norm from a geological point of view,” explained Allmendinger.

While human activity has had many effects on the Earth, speculation suggests that human intervention can actually reduce the magnitude of future earthquakes. Micro-size earthquakes are produced every time a reservoir is created because an enormous weight is placed on the Earth’s surface.

“The water seeps down into the cracks and pores of Earth’s surface and makes the rocks weaker,” Allmendinger said.

The combination of increased pressure and a weakened geological system creates enough stress for magnitude 2 or 3 earthquakes. It has been proposed for humans to induce small earthquakes within gaps, rather than waiting for a big earthquake.

Allmendinger related that no “terribly serious speculation” exists for this idea.

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